Photoexcited dry cleaning processes use highly purified halogen gas, such as chlorine gas, for ultraclean surface-treating of the surfaces of ultra large scale integrated circuits (for example, to remove metallic contaminants from the silicon surface). For this purpose, as diagrammatically illustrated in FIG. 1, which corresponds to FIG. 1 of an article entitled "Photoexcited Processes for Semiconductors II: Dry Cleaning and Dry Etching," by Y. Sato et al, Fujitsu Sci. Tech. J., 27,4,pp. 3-7-328 (Dec. 1991), the halogen gas that has been introduced into a chamber containing a (silicon) wafer to be treated is disassociated into highly active atomic halogen radicals by irradiating the gas with ultraviolet light. These reactive halogen radicals then react with the silicon and metal contaminants of the surface of the wafer to form metallic and silicon halides. Because the silicon wafer is exposed to the photoexciting ultraviolet light, its surface temperature increases, causing the contaminating metallic halides to volatilize (vaporize and lift-off), thereby purging the surface of the wafer of the metallic contaminants. Unfortunately, the silicon halides also volatilize off the wafer surface, causing unwanted etching of the underlying silicon, which leaves the surface of the wafer in a toughened condition that can be detrimental to device performance.
Efforts to limit this undesirable reaction between the halogen radicals and the silicon, such as described in the above mentioned Sato et al article, have included pretreating the surface of the wafer with selected solutions, in order reduce surface etching at elevated temperatures. Still, the silicon-halides are prone to volatilize, causing the wafer surface to be etched and left in a roughened condition.